Set default add_to_movie = 0.0001

[ethanbb]

Description

Partially addresses #1533 - clarifies the behavior of save_memmap by pulling the 0.0001 that is currently added in almost all cases into the default value of add_to_movie. See the issue page for more discussion - we are still considering adding more robust checks for negative or invalid values.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would break back-compatibility)

Technically this could be a breaking change in 2 edge cases:

• The input is a list of multiple .mmap files, which are being concatenated without transposing, slicing or indexing dimensions, adding any constant value, removing initial frames, or setting a border. In this case, currently no constant is added, whereas with this change 0.0001 will always be added by default.
• A non-default value is used for add_to_movie; currently, 0.0001 will be added in addition to this value, whereas with this change the value will be used exactly. It probably will not make a difference in most cases and the current behavior is considered a bug.

Has your PR been tested?

I did not run tests on this exact commit myself because I do not have the environment set up to use pytorch, but I did cherry-pick it onto main and run caimanmanager test, and all tests pass.